The present invention relates to rotary torque-transmitting fluid coupling devices, and more particularly to such devices wherein heat is generated as a result of the torque transmission, and the ability to dissipate such heat represents a limiting factor on the torque-transmitting capability of the device.
Although the present invention may be used with various types and configurations of rotary torque-transmitting fluid coupling devices, it is especially adapted for use with coupling devices which operate on the viscous shear principle, and will be described in connection therewith.
Fluid coupling devices of the type to which the present invention relates have found many uses, one of the most common of which is to drive the cooling fan associated with the radiator of a vehicle engine. Such coupling devices are frequently referred to as "viscous fan drives" because the coupling utilizes a high-viscosity fluid to transmit torque, by means of viscous shear drag, from an input coupling member (clutch) to an output coupling member (housing) to which is bolted the cooling fan.
This invention is especially advantageous when used on a relatively high-torque viscous fan drive, i.e., a fan drive which is capable of transmitting to the cooling fan in the range of about 2 horsepower to about 12 horsepower. Typically, such high-torque or high-horsepower fan drives include an output coupling assembly comprising a cast aluminum housing and a die cast aluminum cover. The input coupling member and the die cast cover normally define a plurality of interdigitated lands and grooves which define the viscous shear space. When this shear space is filled with viscous fluid, torque can be transmitted from the input member to the output assembly.
The torque-transmitting capability of a viscous fan drive is determined by three primary factors: the total viscous shear area; the mean radius of the shear area; and the viscosity of the viscous fluid. Theoretically, the torque-transmitting capability of the viscous fan drive can be increased simply by increasing any one or more of the above-noted factors. However, this approach to increasing torque capability results in certain disadvantages. For example, increasing the viscous shear area, such as by increasing the number of lands and grooves, has conventionally meant increasing the overall size, weight, and manufacturing expense of the fan drive. The same would be true with regard to increasing the mean radius of the viscous shear area. Increasing the torque capacity by increasing the viscosity of the fluid is not a satisfactory solution because it becomes more difficult to pump fluid from the operating chamber to the reservoir chamber as the fluid viscosity increases.
During torque transmission, a substantial amount of heat is generated as a result of the shearing of the viscous fluid between the interdigitated lands and grooves. The amount of heat generated is approximately proportional to the "slip" speed of the fan drive, i.e., the difference between the speed of the input coupling and the speed of the output coupling. As is well known to those skilled in the art, the ability to transmit torque is limited by the ability of the viscous fan drive to dissipate the heat which is generated. If the temperature of the viscous fluid exceeds a certain predetermined maximum temperature, the result will be a deterioration in the viscous properties of the fluid, resulting in a loss of torque-transmitting capability of the drive. Conversely, as the heat-dissipating capability of a viscous fan drive is increased, and the viscous fluid is maintained at a lower temperature, it is possible to achieve a required torque-transmitting capability by means of a smaller, lower cost fan drive. Therefore, those working in the viscous fan drive art have been attempting for many years to increase the heat-dissipating capability of the various fan drive designs.
As was mentioned previously, simply increasing the size of the fan drive and the resulting shear area and mean radius of the shear area is not always a desirable or practical way to increase the torque-transmitting capacity of a viscous fan drive. In addition, there are many situations and applications wherein "packaging" limitations within the engine compartment require that the fan drive not exceed a maximum, predetermined radius. Therefore, in such applications, it is important to be able to increase the torque-transmitting capability of the viscous fan drive without exceeding the maximum, predetermined radius of the unit.